BERLIN — The German capital was on high alert Tuesday with one or more attackers still at large in a deadly truck assault on a Christmas market, an act claimed by the Islamic State that struck at the heart of Europe’s Christian traditions.

Chancellor Angela Merkel decried Monday’s assault — in which a truck carrying a payload of steel plowed into festive stalls and fairgoers in Berlin, leaving 12 dead and dozens injured — as a presumed “terror attack,” even as German police scrambled to find the culprit. The only suspect so far — a Pakistani asylum seeker taken into custody shortly after the bloodshed — was released by police Tuesday because of insufficient evidence.

Late Tuesday, the Islamic State, through the affiliated Amaq news agency, claimed that the attacker was a “soldier” responding to its call to target nations fighting the group in Iraq and Syria. The Islamic State has wielded the term before to describe lone wolves inspired by its rhetoric, and its level of involvement, if any, in coordinating the attack remained unclear.

Italy said it would ramp up security for Christmas events, including Pope Francis’s appearance at St. Peter’s Square in Vatican City. The Czechs pledged “massive” security at public events on Christmas and as the country rings in the new year. French officials said security at Christmas markets had been immediately reinforced, as its lawmakers observed a minute of silence for the all-too-familiar tragedy in Germany.

In Berlin, meanwhile, the release of the only suspect left police scrambling for fresh leads in the assault. They accelerated efforts to study forensic evidence, including analysis of bloodstains within the cabin of the truck — turned into a weapon in a tactic used five months earlier in a similar holiday rampage in Nice, on France’s southern coast.

Investigation teams moved to piece together what they described as “circumstantial evidence,” including witness descriptions and video footage. But no criminal sketches were released to the public, suggesting how much remained unknown. And as night settled over the German capital, Berliners were cautioned to stay on guard.

“It is the case that we possibly still have a dangerous offender in our area,” warned Berlin’s police chief, Klaus Kandt. “These days it is necessary to be vigilant.”

The modus operandi and target, officials said, indicated — but offered no confirmation — that Islamist extremists were involved.

The Islamic State has previously cited traditional Christmas markets as viable targets in their wave of terrorism in Europe, and the Berlin assault was reminiscent of the truck-on-sidewalk tactic used by a self-proclaimed Islamic State adherent in Nice in July. That attack resulted in the deaths of 86 people on the Promenade des Anglais on Bastille Day.

Rita Katz, director of the SITE Intelligence Group, noted that the Islamic State’s claim late Tuesday seemed unique. It is rare for the group to assert responsibility while the perpetrator is still alive. And unlike an official communique — such as those after attacks in Brussels and Paris — its claim through Amaq suggested, Katz said, “a little more distance.”

She said it could indicate that one or more attackers were inspired by the Islamic State but “had little to no coordination with the group.”

German officials were assessing the group’s claim.

“This alleged confession of the so-called Islamic State — in fact they are a terror gang — we only just received,” Interior Minister Thomas de Maizière told public broadcaster ARD. “We should let the security authorities do their work.”

For Berliners, who live in a place often billed as Europe’s most progressive and hedonistic city, the struggle Tuesday boiled down to perseverance — retaining a grip on classic German stoicism in the face of unfolding horror. In a country that eschews displays of patriotism, the colors of the German flag lit up the Brandenburg Gate. Flags flew at half-staff.

Late Tuesday, hundreds of people gathered near the attack site, around Kaiser Wilhelm Memorial Church — a symbol of the rebuilt, post-World War II Germany — to light candles, place flowers and listen to a service broadcast from within the house of worship.

Gabi Meister, a 54-year-old clerk at a car manufacturer, clutched a pink rose with tears in her eyes. “I’m here because I want to show sympathy, because I’m shocked,” she said. “Berlin is an open, cosmopolitan city. We won’t let our tolerance be destroyed by this. . . . We all have to stand together now even more so, all religions, all cultures.”

Merkel — who laid white roses at the attack site in the normally bustling plaza at Breitscheidplatz, in a chic shopping district in west Berlin — called on Germans not to give into fear as the holidays approached.

“We don’t want to live with the fear of evil paralyzing us,” said Merkel, dressed in black as she spoke in Berlin. “We will find the strength for a life as we want to live it in Germany — free, united and open.”

Earlier, Merkel spoke by phone with President Obama, who pledged U.S. aid in the German investigation. As of late Tuesday, only seven of those killed at the market had been identified — the Polish driver and six Germans.

German authorities, accused of mishandling other terrorism-related cases this year, were under mounting pressure to catch the culprit as questions arose about security measures at the market.

That a threat existed was well known. The State Department issued a specific travel warning to Americans last month citing “credible information” that Islamists were targeting holiday season events. Yet the Christmas market attacked Monday appeared to lack basic protections, such as concrete barriers, to ward off a Nice-like attack.

“We cannot turn Christmas markets into fortresses,” Kandt countered.

By midday Tuesday, German authorities were losing confidence that they had caught the right suspect.

The 23-year-old Pakistani had first arrived in Germany last December before coming to Berlin in February. During their investigation, police raided a refugee shelter housed in Berlin’s old Tempelhof Airport, where the suspect lived. After the raid, officials backtracked until prosecutors issued orders to release him.

“The forensic examinations that were carried out could not prove the presence of the accused in the driver’s cab of the truck at the time of the crime so far,” the prosecutors said in a statement.

But the mere prospect of an asylum seeker’s involvement fueled debate in Germany and beyond over Merkel’s decision to allow in nearly a million migrants last year, many of them fleeing war in the Middle East.

Merkel, acknowledging the possibility that a migrant might yet be involved, said it would be “particularly appalling to the many, many Germans who are actively helping refugees every day and to the many people who are indeed needing our protection and are making an effort to integrate in our country.”

Marcus Pretzell, chairman of the right-wing populist party Alternative for Germany, tweeted: “When will the German legal state strike back? When will this damned hypocrisy finally stop? These are Merkel’s dead!”

The incident occurred as Germans have had to endure a growing threat of terrorism — including two small-scale attacks in July. German officials appeared to bungle at least one major case. In October, Jaber Albakr, a 22-year-old Syrian asylum seeker suspected of plotting a bomb attack on a Berlin airport, evaded police for two days. After being caught, he hung himself in his jail cell, despite being placed on a 24-hour suicide watch.